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Mendelian inheritance and Punnett squares

Gregor Mendel followed patterns of inheritance in pea plants, allowing him to elucidate the rules of inheritance, which we can now attribute to the behavior of chromosomes during meiosis. Punnett squares can be used to predict the outcome of a cross between two parents. Created by Sal Khan.

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Video transcript

- [Narrator] This is a photo of Gregor Mendel, who is often known as the father of genetics. And we'll see in a few seconds why, and he was an Abbot of a monastery in Moravia, which is in modern day Czech Republic. And many people had bred plants for agricultural purposes for hundreds, if not thousands of years before Mendel, but he really gave us a glimpse, gave us insights in how traits are really passed. And he did this through his pea plant experiments that he conducted from 1856 to 1863. Over that time period, he bred roughly 28,000 pea plants in order to get a better understanding of how they passed down different traits. And he studied things like properties of the seeds, properties of the pea prods, and things like the height of the plant. And in his time, the mainstream theory was that if you bred a tall parent with a short parent, you would get a medium offspring, but that's not what Mendel saw. When he bred tall pea plants with short pea plants, all of the offspring were tall. But then when he self fertilized those plants and plants have the interesting property that they can fertilize themselves. So the same plant can contribute both the female gamete and the male gametes. In other words the same plant can be both the female parent and the male parent. Well, then he saw that roughly there was a ratio of three to one, tall to short. Those weren't the exact numbers, but pretty close to three to one. And so there's a lot of really interesting things here. First of all, at least for this trait, he didn't see any blending occur. And then the other thing is, this short trait reappeared in this second generation. In order to explain these results, he hypothesized that there are inheritable factors that are inherited from an organism's parents and they're related to a specific trait. So in this case it would be around height. Now we know what he called these inheritable factors. We now call genes, although he did not use the term, and he also hypothesized that these factors could come in different versions. Now, today we know that the different versions of a gene, we call alleles, although he did not use that term, but in this case, the versions that we have at our disposal, you could have a tall height, which we can shorthand say, capital T, capital T for tall, or you could have a short height, which I will use lowercase t for, for a short height. Now generally speaking, organisms will have two versions of their genes like this. For example, an organism could have two tall alleles, or two short, or one of each, but when it produces its gametes, the sex cells, so the sperm for a male and the egg for a female, it will generally contribute one of its two versions to its offspring. And this contribution of one allele or the other is known as Mendel's law of segregation. And we can draw, what's known as a Punnett square to depict this. So let me draw a little bit of a grid here. And Mendel actually did not invent the Punnett square, although he was thinking in these terms. It was actually invented by Reginald Punnett in 1905. And this is useful to think about the probabilities of various combinations based on what each parent could contribute. So let's say we're talking about the tall plant, and let's say it has two tall versions. So it can contribute a capital T or a capital T. And let's say that this short plant over here has two of the short versions for now. So it could contribute either a lowercase t, or a lowercase t. And so what are all the possible combinations for its offspring? Well, in one scenario, you could get this capital T from the male parent, and this lowercase t, from the female parent. In another scenario, you could get this capital T from the male parent, and a lowercase t from the female parent. In this scenario, and I know these look very similar, a capital T from the male parent and this lowercase t from the female parent, and then last but not least, I know this looks repetitive, you could get this capital T from the male parent and this lowercase t from the female parent. And the reason why in all of these cases, you see a tall plant, is because the tall version, and he coined this term, is dominant. And once again, this was all his hypothesis to explain his results. So this is dominant and the short is recessive. So even if you have one of each, you're actually going to show the dominant trait. We now call that your phenotype, what you show is going to be tall. Now, what's interesting about this hypothesis is it seems to explain what happens in the next generation. Just so a little bit of notation. The first generation is usually called the P generation for parental, and then the first generation of offspring is known as the F1, F for filial. And that comes filials, which means sun in Greek, and then the generation after that would be F2, that's just a little bit of notation there, but let's think about what would happen at the F2 generation, if you self fertilized, some of these characters right over here. Well, in that situation, let me draw another Punnett square, on the male parents side, you could contribute either your capital T version, which we now call your dominant allele, or you could contribute the lowercase t version, and on the female parents side and once again for a plant, you can have the same plant that has both the male and the female parent. You could contribute the dominant version, the capital T, or the recessive version, the lowercase t. Now we see something interesting happen in the offspring. There is a one in four chance you get both capital Ts. So capital T, capital T. There's also a one in four chance that you get a lowercase t from the male parent up here, and then you get a capital T from the female parent. There is another one in four chance you get a capital T from the male parent, and a lowercase t from the female parent. And then there's a one in four chance that you get two lowercase ts, one each from the male and female parent. Now, if we accept the dominant and recessive hypothesis, we would expect that plants that got both capital Ts would be tall, but we would also expect that these over here would be tall as well, because the capital T is dominant. They would exhibit the tall phenotype. And then you would expect probabilistically that one fourth of your plants over time, would be short because they only have the recessive alleles, the recessive traits in this situation. And that's actually what Mendel saw. Now what's amazing is that Mendel was able to figure this out without knowing about chromosomes, without knowing all that we know today. Today we know this works because we have 23 pair of chromosomes, and each of those pairs have copies, have different versions of usually the same gene, and that when meiosis occurs and you have gamete formation, one member of each pair will segregate randomly into the newly formed sex cell, into the sperm or the egg. That's why this occur and we go into some detail on that in other videos, but it's pretty cool, that Mendel was able to figure this out in the 19th century.